There is no doubt that directory backlinks can provide a nice boost to your sites rankings, particularly if the directory has a high Page Rank (PR).
Of course the downside is that it can be quite time intensive to obtain high quality directory backlinks. The question remains, is it a worthwhile undertaking?
If you are trying to do the listings yourself, without any specialised software tools to make the submission process faster, then the answer is definitely no. Your time is far better spent promoting your site via other methods.
Of course if you have tools that allow for semi automated submissions then yes, directory submissions can provide a ranking boost. The quality of most backlinks is poor but in terms of link diversity they are useful. They will seldom create any direct traffic; they are purely to be used as a backlinking mechanism.
There are a couple of notable exceptions to this rule, specifically the Yahoo Directory (PR 9) and the DMOZ.org Directory (PR 8). The PR of these two is obviously huge but it is the SEO authority that they bring to your site that is their major benefit. Both of these directories are considered to be ranking factors in the Google algorithm and the benefit of their link is far greater than just their Page rank.
Of course to be listed in the Yahoo Directory currently costs $299 a year and there is no guarantee they will accept you. Probably not recommended for your one page Acai site!
DMOZ.org on the other hand is a free directory but they have a rigorous approval process (read SLOW) which may take many months for your site to be approved, assuming you meet their guidelines. For most affiliate type sites approval is unlikely, unless the site can demonstrate some level of authority.
Still if you have the budget or the patience, then these two directories can make a marked difference in your sites ranking and authority.
